BGs comments are interesting and I don't doubt genuine. It does lead me to a few interesting thoughts though.
Firstly, what was this individual really expecting the pay to be like on MF? Had they not done the slightest research into the projected income? Coming from a temp contract she must surely have been aware that the big allowances were gone for good? After all the raison d'etre of Mixed Fleet was that it was lower cost than existing fleets. Whilst a lower income than the temp contract is undesirable, it surely couldn't come as a surprise?
Secondly, on the matter of service, yes, service is slow. It took over 3 hours to complete the service in Club on the flight I paxed on. But it is a new fleet, and SF LGW took a long time to complete Club service when they started out. They don't know. Practice may not make perfect but it does make efficient.
Finally, the rostering. I don't know if the temp was on WW, where 4 days off after a long trip is assumed, but complaining about getting a long haul trip either side of leave elicits little sympathy from me I'm afraid. This is after all a 365 day industry, and if you want an extra day off then really you need to book that as leave, because if it ain't leave then it's the companys day. On long haul we all learn to make the most of our time off and part of that is working through the tiredness. If the individual concerned can't enjoy a day off because she's tired the day after a return from the trip then regret to say long haul really isn't for her and she won't cope in the long run.
Whilst I am sorry that the girl concerned isn't enjoying Mixed Fleet despite her initial enthusiasm, it does strike me that her expectations are a little unrealistic. I suspect this may have been partly what BA had in mind when they were concerned about contaminating Mixed Fleet; not that the people were bad, but that their expectations were unrealistic.